Rule
A style which is made up of a selector and a declaration is called a rule.
Selector
A selector determines which elements are affected by the declaration. It identifies which elements you wish to format.
Declaration
The declaration is made up of one or more property/value pairs, which specifies what should be done.
Property and Value
Property is the name of the CSS property that describes the sort of formatting you’d like to apply and value is one of a list of allowable options for that property.